1. Field of the Invention
This invention relates to rodded open lattice nuclear fuel assemblies and more particularly provides a fuel assembly having fuel rods of differing diameter.
2. Description of the Prior Art
Nuclear reactors typically include a core made up of a plurality of nuclear fuel assemblies disposed within a vessel through which a gaseous or liquid coolant flows. The coolant removes energy in the form of heat as it flows about and through the fuel assemblies. Particularly in pressurized reactors circulating a liquid coolant such as water it is undesirable to reach a boiling condition and, accordingly, reactor cores are designed to prevent a condition commonly referred to as "departure from nucleate boiling" (DNB). If a vapor is permitted to exist along the fuel rods the heat transfer from the heat generating rods to the surrounding coolant is substantially impaired and a potenstial exists for damage to the fuel rods through overheating. Accordingly, DNB related safety criteria impose an upper boundary on the maximum coolant temperature and therefore limit total reactor efficiency. These concerns are further complicated by the desired utilization of mixed oxide or plutonium fuels which are more expensive to fabricate and inherently have a high neutronic capture cross section and a strong moderator temperature coefficient as compared to more commonly used uranium fuels. And, since the coolant becomes progressively hotter as it flows upwardly through the core it therefore progressively changes density, providing progressively less moderation at the upper portion of the core as compared to the bottom. This results in less control and design flexibility.
Approaches toward alleviating these limitations, primarily in response to DNB effects, include increasing the water-to-fuel ratio of the core and utilizing full core length fuel rods of smaller cross section which generate less energy per unit length. These approaches, however, are complicated by other factors including excessive fabrication costs and larger cores. Additionally, reactor designs incorporating redundant safety systems which respond to the unlikely event of a rupture of the reactor coolant system causing a loss of coolant, flood the core area with coolant that builds up from the bottom toward the top of the core. Additional flexibility in providing a core design responsive to this condition is advantageous.
It is therefore desirable to provide a nuclear fuel assembly which alleviates the temperature and efficiency limitations, and it further is desirable to provide such an assembly which is compatible with mixed-oxide fuel utilization. It is additionally desirable to provide design flexibility to respond to the varying coolant density throughout the core and unlikely coolant discharge occurrences.